I will be the first to admit I have absolutely no engineering knowledge whatsoever, but it just seems to me intuitively that it would be cheaper to just build a new stadium with a roof then try to retrofit an old outdoor stadium with a retractable roof. Has that ever been done anywhere?

"Genius is not replicable. Inspiration, though, is contagious, and multiform — and even just to see, close up, power and aggression made vulnerable to beauty is to feel inspired and (in a fleeting, mortal way) reconciled."
--David Foster Wallace, () "Roger Federer as Religious Experience"